Revisiting Clements and Gleason: Insights from Plant Distributions on Pikes Peak, Clements’s Life-Long Study Site

AbstractHow do species' distributions respond to their environments? This question was at the heart of the Clements-Gleason controversy, ecology's most famous debate. Do species respond to the environment in concerted ways, leading to distinct and cohesive assemblages (the Clementsian paradigm), or do species respond to the environment independently (the Gleasonian paradigm)? Using plant occurrences along the elevation gradient of Pikes Peak (Colorado) as a lens through which to gain insight into Clements's perspectives on the debate, we formally test for community patterns along this gradient using a modern framework unavailable at the time of Clements and Gleason. The Pikes Peak region was Clements's study area for more than 40 years, where he established a research lab and distributed sites along the elevational gradient. His investigations of plant distributions on this mountain likely influenced his views on communities. We found mixed support for the paradigms, with neither the Gleasonian paradigm nor the Clementsian paradigm fully supported. While distributions along the gradient showed evidence of clustering of species range edges, considered to be consistent with the Clementsian paradigm, the pattern was weak, and neither range edges nor species turnover peaked at ecotone elevations, as expected under the Clementsian paradigm. Our results illuminate the Clements-Gleason debate by allowing us to probe issues that complicate conclusively testing the paradigms, such as deciding on how we quantify environmental gradients and determining the appropriate scales for community patterns and processes that might generate them. Revisiting the debate also revealed that Clements's and Gleason's views had more in common than we realize. The debate may be less neatly resolved than we assume from mythos, and it continues to have relevance to basic and applied ecology today, as its legacy has shaped our (still tenuous) notion of ecological communities and the trajectory of our field.